Nitrocellulose manufacture and cellulose therefor



Patented Sept. 27, 1932 UNITED STATES PATENT OFFICE -MILTON O. SCHUR BENJAMIN G. B008, 013 BERLIN, NEW HAMPSHIRE, ASSIGNORS TO BROWN COMPANY, OF BERLIN, NEW HAMPSHIRE, A CORPORATION OF MAINE 'NITBOCELLULOSE MANUFACTURE AND- CELLULOSE THEREFOR ms Drawing,

The subject of this invention is nitrocellulose manufacture and cellulose intended for such purpose, one of its primary objects being to provide cellulosic material in a form 5 making for uniform and-high quality 'nitrocellulose products. 7

It is the practice to nitrate cellulose fiber in a variety of physical conditions. In' the case of chemical wood pulp, it is sometimes preferred to nitrate the pulp as it is purchased, for instance, in the form of thick sheets of dry pulp, known in the trade as .drier sheets; Such sheets may be nitrated by steeping in suitable nitrating acid compositions, and may be kept'in integral condition throughout the nitration and afternitration treatments. They constitute a desirable. compact form of material, easy to handle.

The pulp manufacturer produces drier sheets on a machine of the papermaking type, comprising a wet'end, to which the pulp is delivered and sheeted on a mould or Fourdrinier wire, and a dry end or series of drum driers, over whichthe sheet passes and is progressively dried. The sheet is usually madeas compact as is possible, by press rolls at the wet end of the machine acting to squeeze the maximum amount of water from the freshly-formed web, since otherwise there is added steam i consumption "and/or a reduction "in the capacity of the machine. In fact, drier sheet as heretofore made normally had a compactness of about 70. The

compactness value herein given represents the ratio of basis weight to 100 times the thickness of-sheet in inches. The basis weight is the weight in pounds of 415,000 square inches of the sheet. According to this definition of compactness, the compactness value of a sheet having a basis weight of 240 pounds and an average thickness of .040 inches will be In our work on the nitration of drier sheets, we have discovered that for best and for consistent results it is essential to control the compactness of the sheets and pref- .Application filed April 20,

1931. Serial m. 531,638.

erably also the basis weight of thickness of i the sheets. Thus, we have found that if the compactness is too high, the pores or void spaces in the sheets are so small that penetration of the quite viscous nitrating acid into and through the sheets, as well as the diifusion of the acid from the sheets'into the main body of nitrating acid is greatly hindered, with the resultthat complete ni- '7 tration' of the sheets is prolonged and there is a tendency toward hydrolysis and other side reactions, with accompanying loss in yield and impairment in quality of the prod,- uct. On the other hand, if the compactness is too low, retention of the expensive nitrating acid by the sheets becomes excessive; and the nitrating reaction may proceed so quickly as to generate heat to a point where hydrolysis and other side reactions also become excessive, especially when no provision is made to cool the batch undergoing nitration. From the standpoint of handling, the basis weight or thickness of the sheets is a factor of great importance. For instance, if

- the basis weight is too low, it is expensive to tend to move around in the bath of nitrating acid, as well as in the subsequent drowning,

washing, and stabilizing baths, On the other hand, if the basis weight is too high, an inordinate time periodis required for nitration of the sheets throughout; and this means tieup of nitrating acid and nitrating equipment,

and low-capacity production.

After considerable experimentation, we have established that optimum results are realized when drier sheets having a compactness between about 40 and 7 O constitute the raw material undergoing nitration, particularly'when such compactness is coordinate with a sheet thickness of about .040 to .070 inches. The sheets employed are preferably composed of refined wood pulps :of high whiteness and high alpha cellulose content,

sible. Owing to the substantially unbeaten such as are adapted for conversion into nitrocelluloses of a quality on a par with those derived from cotton; These wood pulps, in accordance with usual practice, may be used 'in substantially unbeaten condition in the formation of the waterleaf sheets-(i. e. sheets free from sizing materials or fillers). In the case of hardwood pulps e. g., birch, maple or poplar, the compactness of the sheets with in the range specified may be somewhat highof the sheet, the substantially unbeaten pulp i the intensity of which is greatest, of course,

calities in the sheet where the compactness is ,where the layer is densest.

is generally picked up as a layer on a cylinder mould, and is then passed through pressrolls which clewater the layer as far as poscondition of the fibers, they arelaid down wildly or as fiber aggregates, with the result that when the layer is passed between the press-rolls, it receives an uneven pressure,

This produces local areas in the sheet of very high compactness, so that, although the average compactness of the sheet is, say, 70, there may be loconsiderably greater. This variation in local compactness of the sheet should be combat-ted byan intelligent operation of the wet end of the pulp drier, or be ofl'set'by alower average compactness in the case of soft-wood pulps than in the case of hardwood pulps. The use of large volumes of water in the. vat of the cylinder mould along with the pulp, or the use of a Four-drinier wet end, is of assistance inattaining uniform formation in the sheet, even though there is atendency for soft-wood pulps to form wilder sheets than hardwood pulps.

The simplest method for controlling corn pactness to produce sheets such as we desire,

is through adjustment of the pressro lls at the wet end of the machine to exert less pressure on the wet sheet. Heretofore, as already'observed, it has been the practice to exert as much pressure as possible on the 'wet sheet, without crushing it, in order that the amount of water retained by the sheet entering'the dry end of the machine be a minimum.- The inevitable result of the application of such high pressure is a drier sheet of relatively high compactness, usually'above 70. We have found, 'however, that in the case of drier sheet intended for conversion Into n1trocellulose,-the higher steam consumption and/or loss ofproduction ca'pacityresultin'gfrom the exertion of less pressure on the wet sheet is more than compensated for by the benefits received in the nitration process. The raising of the press-rolls on the.

pulp drier can without trouble be made such that a dry sheet having a compactness between about 40 and 70 comes ofi' the machine. Specifically, we prefer to produce drier sheets of a compactness of about 50, although this compactness value may be departed from within the range specified,while still being within the ambit of the present invention.

' The improvement in the nitrating process realized by following the principles of the present invention obtains ,in the case of various compositions or nitrating acid. For example, we have performed nitrations in typical nitrating acid whose composition was 18% water, 22% nitric acid, and sulphuric acid. When using drier sheets having a compactness of 48 and a thickness of .050 inches with such acid, the results obtained were notably better than those sew cured under similar conditions with drier sheets having a compactness of 75v and a ,thickness of .050 inches, or with drier sheets having a compactness of: 7 5 and a thickness greater or less than .050 inches. In fact, the nitration performed with our sheets of lower compactness were completed sooner. Important, too, was the fact that'the nitrated sheets obtained from our drier sheets of lower compactness lend themselves, after washingand stabilizing, to easier dehydration and to subsequent disintegration" and solution or colloidization. Evidently, the greater porosity inthe sheets endures after the nitrating operation and permits a more rapid displacement of the water in the sheets by alcohol, as well as a more rapid penetration of the solvent or colloidizing agent. Or, when the nitrated sheets -are disintegrated, as in a beater, they are less resistant to being pulled apart into shreds or pulp. Theforegoing advantages also accrue when drier sheets of a compactness of 48 and a thickness of .050 inchesare subjected to other nitrat- --i-ng processes, for 1nsta'nce',"the multiplestage nitration process disclosed in application Serial No. 451,995, filed May 13, 1930, by Milton 0. Schur. In'such case, the firststage nitrating acid may have asulphuric to nitric acid ratio of less than 2 to -1, and the second-stage nitrating acidmay be one in which this ratio is greater, as in the usual nitrating acid. Thus,'the primary-stage niwater, 40.6% nitric acid, and 40.7% sulphurvtrating acid may be composed of 18.7%

ic' acid; and the secondary-stage nitrating j acid may be composed of 18.7% wat'er, 20.5% nitric acid, and 60.8% sulphuric acid. The nitrocelluloses thus produced yield solutions of remarkable clarlty and freedom. from unnitrated fibers.

Our invention has its greatest utility, so

far-as we are now aware, in the field of chemical wood pulps, and more especially wood pulps of high whiteness and alpha cellulose content intended for nitration purposes. Under certain economic conditions, it may be permissible to mix in a certain percentage of cotton linters with the wood fibers to make easier the attainment of low compactness sheets, for cotton linters tend to keep a sheet of mixed fibers bulky and porous. .W'hen in the appended claims we speak of wood pulp or Wood fibers, we, therefore, mean such fibrous material alone or blended with a subordinate proportion of, say 5% to 10%, more or less, cotton fibers.

What we claim is:

1'. A process which comprises nitrating sheets of cellulose pulp whose thickness lies between .020 inches and .070 inches, and whose compactness in terms of ratio of basis weight to times the thickness in inches lies between 40 and 70.

2. A sheet of interfelted wood fibers of high alpha cellulose content whose thickness lies between .020 inches and .070 inches, and whose compactness in terms of ratio of basis weight to 100 times the thickness in'inches lies between 40 and 70.

3. A sheet of substantiallyunbeaten, interfelted wood pulp fibers of high whiteness and alpha cellulose content, whose thickness lies between .020 inches and .070 inches, and whose compactness in terms of ratio of basis weight to 100 times the thickness in inches lies between 40 and 70.

In testimony whereof we have aflixed .our

signatures.

MILTON O. SCHUR. BENJAMIN G. HUGS 

